Protecting Wooden Sleepers
Railway sleepers are traditionally protected with a coal-tar based chemical, which has been subject to tremendous development in recent decades. Although initially very high, toxic content has been reduced considerably. Earlier formulations contained up to 500 ppm of benzo(a)pyrene, while the concentration of the same is below 50, and actually between 5 and 15 ppm in modern oils, such as WEI-C and WEI C GX-Plus. Despite the development of the impregnating agent (creosote) used in the treatment of sleepers and poles, the use of creosote and creosote-treated wood has been strictly restricted. These restrictions are set forth in Directive 94/60/EC and then again in Directive 2001/90/EC. The need to introduce stricter regulations arose when research justified that the carcinogen effect of creosote is larger than the earlier indicated. The tightening of the regulations culminated in Commission Directive 2011/71/EU, which classifies creosote as a biocidal product. The use of creosote-treated wood is still allowed under the terms provided by the EU and will be allowed in EU member states until 31 April 2018 unless otherwise provided. Its use after that date may depend on research results achieved by 2018, i.e. the viability of alternatives developed to replace creosote as a preservative of sleepers. Important as it is, this issue is not to be neglected as the ratio of wooden sleepers is extremely high in sidetracks (Figure 1) and switches in the member states of the European Union.
There are several completed studies and research is also still in progress about potential creosote replacements. The most accurate summary of the results has been published in a study on Sustainable Wooden Sleepers (SUWOS), which relies on surveys and the results of consultations conducted by the International Union of Railway (UIC) with organisations of the wood sector and infrastructure managers. The Institute of Wood Studies at the University of Western Hungary (IWS UWH) has been involved in research into protecting wood and wood as a substance. Also, IWS UWH has been an accredited organisation engaged in rating and taking receipt of sleepers used in railway tracks in Hungary. We are evaluating and expressing an opinion about the alternatives recommended by the study for the replacement of creosote based on the above, as well as on decades of experience with wood sector research, primarily wood protection, and with sleeper rating.
Using Alternative Protective Agents
Among the main obstacles of using substances that have been proven successful in other areas, the ability to impregnate large cross sections, the durability of protection, leach out properties, and utilisation after removal from the track are worthy of mentioning.
New materials have surfaced but these are still subject to research, including vegetable oils, which pose problems of durability. Research conducted by the Institute of Wood Studies suggests that vegetable oils offer no protection whatsoever for wooden materials against fungi or insects.
Modifying Wooden Material
Treatment with dry heat would be an effective method for improving durability by protecting wood against biological attack, but it is extremely costly and time consuming because of the large diameters. Moreover, thermally treated wood loses much of its flexibility, which is exactly the property that renders wood suitable as a substance for sleepers. Furthermore, certain solidity features also deteriorate, especially during treatment under high temperatures. Weatherproofing is another problem as thermal treatment fails to offer proper protection against harsh weather conditions.
Heat treatment in oil promises somewhat more favourable results than dry treatment. Although the compressive strength of wood increases, its strength to withstand forces perpendicular to wooden fibres diminishes. Most of the flexible properties also show adverse changes.
On the other hand, both procedures definitely improve resistance to biocidal attack, but heat treatment drastically reduces the resistance of wooden substances to dynamic stresses, about 50-60% of which is lost during after treatment. As a result, standards have forbidden the use of such modified wooden substances in civil engineering.
Modification with Chemicals
Acetylating is only a partial solution for increasing resistance to biological attack. The main problem is that the effect is short-lived, particularly in Hazard Category 4. Large cross sections are also almost unmanageable. No actual tests of the solution have been performed under real life conditions.
The testing of DMDHEU treatment is still in an extremely early stage, nevertheless it is not exactly promising in terms of cost and the stability of the compound.
Sleepers Made of Alternative Materials
Steel is also used in limited quantity as a substance for sleepers, but the ratio of steel sleepers is expected to decrease mainly because of their extremely unfavourable ecological performance compared to wood.
Developing composites of wood and plastic could be an interesting direction of research. This way, flexibility benefits may be preserved and resistance to moisture improves along with biological durability. The quality of reaction to climatic effects (UV radiation, major swings of temperature, etc.) is still a major challenge, and the stability of plastics selected for the purpose is also a problem.
Using Untreated Wood
Using pine varieties without treatment is clearly infeasible due to the brevity of lifetime. Neither fir nor redwood offers proper solutions without the use of a protective agent. The use of exotic wood in sleepers occurs sporadically today. The most frequent variety is Azobe wood. Application in large volumes to meet the demand for sleepers in Europe is practically impossible mainly due to the character and position of forest management in Africa and because of costs.
The SUWOS study puts the durability of beech and oak essentially in the same category. Reality, however, suggests otherwise. Beech is classified by the EN 350-2:1998 standard in Category 5 (not durable), whilst European oak is listed in Category 2 (durable). Literature also estimates the lifetime of European oak at 25-30 years outdoors without treatment with chemicals. Practice justifies the same, and it is not by chance that bridge-sleepers are used in structures without treatment in Slovenia. When wood is encased in soil, soil type is a major factor in terms of durability. If soil type is beneficial, the lifetime of good quality European oak varieties may be extended substantially (Figure 2).
The discussion above suggests that wooden sleepers, which are currently treated with creosote, are indispensable and the ratio of use is likely to increase. Naturally, research must continue to find a potential solution to mitigate environmental load and the toxic effects that harm human health.•